I would lead in with G46 or G47 to a point on one of the straight sides of the triangle. The lead in radius would need to be small enough to avoid clashing the cutter with the opposite sides of the pocket.

I would G41 G46 A_._ to the midpoint of one side, move CCW around the triangle at feed rate from vertex to vertex to vertex and back to the starting point, and then G40 G46 A_._. The contour is so simple, you don't need to mess with G91 and G90 to increment the depths; simply type the path again at a new depth if that's what you want. Run the contour once with a big enough cutter to knock out the middle of the pocket, and then run the small cutter to sharpen the corners. Consider running with a radius allowance and then a clean-up pass with no allowance. Consider going around twice with your final pass to account for cutter bending.

First Time CnC, Contour 3 help needed!

Programming the tool to be at the middle prior to downfeed and G41 is safest. If you use M70 the tool will move to the start position automatically. The control determines the lead-in direction based on G41 vs G42. I would not use G42.

There is no error. The control moves the cutter from wherever it happens to be, center or anywhere else, to the start point of the lead-in arc. The control figures out the location of the proper start point based on the given value of A and the target point on the contour. You don’t have to calculate A. Just make it not so big that the cutter clashes with the opposite side of the pocket.

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